Fourier transform infrared (FTIR) interferometric spectrometers are widely used in the analysis of chemical compounds. By measuring the absorption of infrared radiation by an unknown sample at various wave lengths in the infrared spectrum and comparing the results with known standards, these instruments generate useful information with respect to the chemical makeup of the unknown sample. In a typical FTIR spectrometer, infrared radiation from an infrared emitting source is collected, passed through an interferometer, passed through the sample to be analyzed, and brought to focus on an infrared detector. The interferometer system, in combination with the sample, modulates the intensity of the infrared radiation that impinges on the detector, and thereby forms a time variant intensity signal. It is the function of the detector to convert this time variant intensity signal to a corresponding time varying current. The current, in turn, is converted to a time varying voltage, which is presented to an analog-to-digital converter and then stored as a sequence of digital numbers to be processed in a processor associated with the spectrometer.
One important feature of the FTIR spectrometer is the moving mirror element that modulates the analytical radiation used by the instrument to study samples. The moving mirror allows a time-domain interferogram to be generated which, when analyzed, allows high resolution frequency-domain spectra to be produced. The computer performs a Fourier transform on the data to produce a spectrum which shows spectral-energy versus frequency.
It is critical in the design of these instruments that the surface of the moving mirror be very accurately held in an orthogonal position, i.e., at a right angle, both to the fixed mirror and to the direction of the motion of the moving mirror. Mirror positional accuracy is crucial because deviations in the mirror alignment produce small errors in the time-domain interferogram which may translate into large errors in the frequency-domain spectrum. In a typical interferometer, mirror deviations larger than one wave length of the analytical radiation used are considered significant and can seriously degrade the quality of the entire instrument.
The alignment of the mirror is ordinarily accomplished by supporting the mirror in a high precision bearing, such as an air bearing, and by attempting to align the bearing to the desired degree of precision. Alignment is usually accomplished by means of differential screws which are manually adjusted to align the moving mirror as perfectly as possible. This is a time consuming procedure requiring significant skill. It also adds to manufacturing expense and to field service costs because realignment is often required. In addition, it mandates the use of extremely accurate bearings which may be very expensive.
Efforts have been made to eliminate the need to manually align the high precision bearings. Although still requiring the use of high precision bearings, automatic static alignment at least relieves the user from performing time consuming realignments. For instance, some devices which automatically align the moving mirror use stepper motors to accomplish substantially automatic simulation of the manual alignment procedure. These devices typically use a computer which aids in the alignment of the static mirror at periodic service intervals. Disadvantages of these devices include slow speed, large size, high cost, and continuing dependence on high precision bearings.
Attempts to eliminate the high precision bearings have heretofore been only marginally successful. To attempt to dynamically tilt either the moving or the fixed mirror to compensate for the tilting of the moving mirror as it moves on its air bearing requires more speed than can be readily obtained with a mechanism based on lead screws and stepper motors.
Dynamic adjustment of the mirror tilt to correct for imprecise bearings and achieve desired alignment has been difficult to accomplish in practice. Such prior adjustment devices tend to be very expensive, slow, bulky, and poor in performance. For example, one device uses piezoelectric positioners to dynamically adjust mirror tilt. However, the positioners are large, expensive, and require one thousand volt drive levels. In addition to being large and expensive, power supplies for such high voltages create undesirable operating hazards.